Photovoltaic systems use solar cells to convert light into electricity. A typical photovoltaic system includes several components, including photovoltaic cells, mechanical and electrical connections, mountings, and controllers for regulating and/or modifying the electrical current produced by the photovoltaic system.
The following terms are used herein to describe various components and/or operational aspects of photovoltaic systems:
PV photovoltaic
DC direct current
AC alternate current
VOC open circuit voltage
VGRID grid voltage
VNOM nominal grid voltage
IGRID grid current
FIG. 1 is a functional block diagram of a typical PV system 100. The photovoltaic system 100 includes a photovoltaic generator 101 that converts sunlight into electricity. In a conventional PV system, such as the photovoltaic system 100, the voltage generated by the system can be controlled by extracting an appropriate amount of power from the PV generator 101, passing the power from the PV generator 101 to a power converter 102 through to a power sink 103. According to an embodiment, the power converter 102 can comprise an electronic power converter. In a typical implementation, the power sink 103 is the electrical power grid (sometimes also referred to as the power “mains”). The grid comprises an electrical network for generating, transmitting, controlling, and distributing power from power generators to power consumers at various service locations across the network. The power converter 102 converts DC power provided by the PV generator 101 into AC power that can be distributed on the grid.
FIG. 2 is a more detailed block diagram of a conventional PV power system 200 that can be used to implement the system illustrated in FIG. 1. The PV power system 200 includes a solar cell array 201 that comprises solar cells (also referred to as photovoltaic cells). The solar cells are solid state devices that convert the energy of sunlight directly into electricity by the photovoltaic effect. The solar cells generate DC voltage.
The solar cell array 201 is coupled to a DC switch 202. The DC switch 202 can be closed to connect the solar cell array 201 to DC capacitor bank 204, or opened to disconnect the solar cell array 201 from the DC capacitor bank 204. When the DC switch 202 is closed and the solar cell array 201 is generating power, the solar cell array 201 can provide power to charge the DC capacitor bank 204. The DC capacitor bank 204 is also connected to an inverter 205.
The inverter 205 converts the DC voltage output from the capacitor bank 204 into a 3-phase (or in some cases 2-phase) pulsed AC voltage. The inverter 205 outputs pulsed AC current to a filter 206. The filter 206 converts the pulsed AC current output by the inverter 205 into a sinusoidal AC voltage. The sinusoidal AC voltage can then be output to a mains power grid 209. If an AC mains switch 207 is closed, the sinusoidal AC voltage output by the filter 206 is received by the power transformer 208. The power transformer 208 adapts the voltage output by the PV system 200 to the grid voltage. This configuration allows the PV system 200 to output electricity onto the mains grid 209. The voltage output by the photovoltaic system 200 is no higher than the grid voltage.
Controlling the voltage generated by a PV generator such as the solar cell array 201 is important because it can help to (a) increase the power generated by the solar panels, and (b) reduce the voltage stress on the power converter. If the power sink 103, such as the grid 209, is unable to absorb the available power produced by the PV generator 101, the PV voltage will increase toward the open circuit level (VOC) and will ultimately produce an increased voltage stress on the power converter 102. In conventional systems, this is addressed by “overdesigning” the power converter, such that the power converter 101 can reliably operate with the PV open circuit voltage levels. Overdesigned systems have lower efficiency and higher complexity than systems that are not overdesigned.
FIG. 3 illustrates an alternative approach that conventional systems have used to address these issues. A PV power system 300 includes a pre-load 304 in the form of a dissipative resistive load parallel to a PV generator 301. In the event that a power sink 303 is unable to absorb the power generated by the PV generator 301, the pre-load 304 can be activated to supplement the power sink and to maintain the PV voltage at levels that are safe for power converter 302. The use of a pre-load 304, however, can be prohibitively expensive and can pose a fire risk.